Understanding　muscle　synergy　variability　and　its　clinical　relevance　in　rotator　cuff　tear　(RCT)　patients　is　crucial　for　elucidating　motor　control　mechanisms　and　informing　rehabilitation.　This　study　uses　non-negative　matrix　factorization　(NMF)　to　assess　the　influence　of　age　and　pathological　factors　on　synergy　patterns　during　abduction　(ABD)　and　flexion　(FL)　tasks.　Fifteen　young　controls　(YC),　fifteen　elderly　controls　(EC),　and　twenty　elderly　RCT　patients　were　recruited.　Surface　electromyography　(sEMG)　signals　from　eight　shoulder　muscles　were　recorded　for　NMF　analysis,　and　the　correlation　with　the　Constant-Murley　Score　(CMS)　was　evaluated　via　regression.　Results　revealed　that　the　number　of　synergies　in　the　FL　task　was　significantly　lower　in　the　RCT　group　compared　to　the　EC　(4.5　±　1.3　vs.　5.6　±　1.1,　P=0.01)　and　YC　groups　(4.5　±　1.3　vs.　5.6　±　1.0,　P=0.03),　with　no　significant　difference　in　the　ABD　task　(P=0.10).　In　ABD,　synergy　pattern　1　was　dominated　by　the　upper　trapezius　and　middle　trapezius,　while　in　the　EC　group,　pattern　4　was　driven　by　the　supraspinatus,　and　in　the　YC　group,　both　supraspinatus　and　posterior　deltoid　were　involved.　In　FL,　the　middle　deltoid　weight　was　significantly　increased　in　pattern　3　in　the　RCT　group,　suggesting　compensatory　activation　or　a　pathological　marker.　The　root　mean　square　value　of　middle　deltoid　strongly　correlated　with　CMS　(ABD:　R2　=0.53,　P2　=0.46,　P　©　2001-2011　IEEE.